Apparatus and method for downhole fluid separation

ABSTRACT

This disclosure concerns a static oil/water separation chamber that is installed in a well extending to an underaround production formation containing hydrocarbon oil and water. The separation chamber has an inlet for receiving well fluid from a section below the separation chamber and two outlets. One outlet discharges a water-enriched component into a discharge well section, and the other outlet produces an oil-enriched component. The height of the separation chamber is larger than the thickness of the dispersion band that is formed under normal operating conditions.

FIELD OF THE INVENTION

The present invention relates to a well for producing oil from anunderground formation. The invention relates in particular to a well,wherein a well fluid is separated underground, such that an oil-enrichedcomponent of the well fluid is produced to the earth's surface. It willbe understood, that the earth's surface may also be the bottom of thesea.

BACKGROUND OF THE INVENTION

International patent application publication No.98/41304 discloses sucha well having a horizontal section that includes the separation chamber.

U.S. Pat. No. 5,842,520 and U.S. Pat. No. 5,979,559 discloses such awell, wherein the separation chamber is located at substantially thesame level as the production formation.

International patent application publication No.98/02637 discloses sucha well, wherein the separation chamber is located at the level of theproduction formation, and wherein the static separator is a cycloneseparator.

U.S. Pat. No. 4,793,408 discloses such a well, wherein the separationchamber is a small-diameter chamber located within a section of the welland having a side inlet for the well fluid, and wherein the separationchamber is provided with regulators for regulating the discontinuouswithdrawal of effluents.

U.S. Pat. No. 5,443,120 discloses a cased well including a separationsection in the casing adjacent the underground production formation,which is arranged for separating of at least a portion of the water fromthe well fluid.

U.S. Pat. No. 5,857,519 discloses a gas lift well including a separatorarranged in the annulus between the casing and a tubing string andadjacent the underground production formation.

The known systems generally suffer from one or more drawbacks, includingan insufficient degree of separation, complexity and high installationcost, limited robustness, limited operation window for oil productionflow rates and watercut.

SUMMARY OF THE INVENTION

In the specification and in the claims, the expression ‘well fluid’ willbe used to refer to a fluid comprising hydrocarbon oil and water.Further, hydrocarbon oil will be referred to as oil. The well fluid canfurther comprise gas.

There is an increasing need for efficient underground separation ofwater from a well fluid. Ideally, the well fluid is separated into oiland water, wherein the oil is sufficiently de-watered such that no orlimited additional separation near the wellhead at the surface is neededprior to transport from the field, and wherein the water is ofsufficient purity to allow injection into an underground formation.

Such a well wherein a well fluid is separated extends from the earth'ssurface to an underground production formation containing hydrocarbonoil and water. The well is provided with a separation chamber in whichan oil/water separator is arranged comprising an inlet to receive wellfluid, an outlet for an oil-enriched component opening into the wellsection above the separation chamber and an outlet for a water-enrichedcomponent opening into a deposition well section below the separationchamber.

It is an object of the present invention to provide a well that allowsefficient, robust underground separation for well fluid intooil-enriched and water-enriched components.

It is another object of the present invention to provide a well forproducing oil from an underground formation to the surface, wherein theoil can be de-watered below the surface, such that the waterconcentration of the produced oil is sufficiently low that no or limitedfurther de-watering at the surface is needed.

It is a further object of the present invention, to provide a wellcomprising an underground separation chamber wherein the feed and theseparated components flow vertically or nearly vertical in and out ofthe separation chamber.

To this end the present invention provides a well extending from theearth's surface to an underground production formation containinghydrocarbon oil and water, which well above the production formation isprovided with a separation chamber in which a static oil/water separatoris arranged comprising an inlet to receive well fluid from an inlet wellsection below the separation chamber, an outlet for an oil-enrichedcomponent opening into the well section above the separation chamber andan outlet for a water-enriched component opening into a discharge wellsection below the separation chamber, wherein the height of theseparation chamber is larger than the thickness of the dispersion bandthat is formed therein under normal operation conditions.

The static separator in one particular embodiment further comprises aflow distributor means, arranged to distribute at a predeterminedvertical position the well fluid received through the separator's inletover the cross-sectional area of the separation chamber. The separatorcan further comprise a level detector means and a flow control means inorder to maintain during normal operation an interface between twoliquid layers at a predetermined level.

In an alternative embodiment, the static separator according to thepresent invention further comprises:

a stack of vertically spaced apart inclined plates, wherein between eachpair of neighbouring plates a separation space is defined;

a substantially vertical inlet conduit communicating with theseparator's inlet, which inlet conduit traverses the stack of plates andis arranged to receive the well fluid at its lower end, and is providedwith one or more well fluid outlets each of which opens into aseparation space;

a substantially vertical oil collection channel having an oil outlet atits upper end communicating with the separator's outlet for theoil-enriched component, which oil collection channel has one or more oilinlets, each oil inlet being arranged to receive fluid from theuppermost region of a separation space, wherein at least the plateimmediately below each oil inlet is provided with a vertically upwardpointing baffle; and

a substantially vertical water collection channel having a water outletat its lower end communicating with the separator's outlet for thewater-enriched component, which water collection channel has one or morewater inlets, each water inlet being arranged to receive fluid from thelowermost region of a separation space, wherein at least the plateimmediately above each water inlet is provided with a verticallydownward pointing baffle.

The expression height of the separation chamber is used in thespecification and in the claims to refer to the shortest verticaldistance between the outlet for the oil-enriched component and theoutlet for the water-enriched component. The physical height of theseparation chamber can be larger.

There is further provided a method of producing oil from an undergroundproduction formation through a well according to the present invention,which method comprises the steps of

admitting well fluid into the separation chamber at a predeterminedvertical position through one or more openings at a local flow velocitybelow 1 m/s;

allowing the well fluid to separate into a lower layer of awater-enriched component, a middle layer of an oil and water dispersioncomponent and an upper layer of an oil-enriched component,

withdrawing liquid from the upper layer and producing this liquid to thesurface;

withdrawing liquid from the lower layer;

measuring the vertical position of the interface between two liquidlayers; and

controlling the flow rate of at least one of the inflowing well fluid,the outflowing water-enriched component or the outflowing oil-enrichedcomponent in dependence on the measured vertical position.

Applicant has found that from a practical point of view it isadvantageous to arrange the separation chamber downstream of, and abovethe production formation, and that for such a configuration it isrequired that the height of the separation chamber is larger than thethickness of the dispersion band that is formed under normal operationconditions. Then, during normal operation a layer of relatively dry oilis formed above the dispersion band and a layer of relatively pure waterbelow the dispersion band.

It has further been recognised that by separating the well fluid in anunderground separation chamber one can take advantage of the physicalconditions in the well, e.g. elevated temperature and pressure, whichinfluence the separation behaviour of oil and water such that efficientseparation of well fluid into relatively dry oil and relatively purewater can be achieved under practically and economically feasibleconditions. According to a specific aspect of the invention, theefficiency of an underground separation chamber can be enhanced by usinga separator comprising a stack of plates.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example in more detail andwith reference to the accompanying drawings, wherein

FIG. 1 shows the result of model calculations of the separation of awell fluid in a separation chamber with and without an installed stackof plates;

FIG. 2 shows schematically a first embodiment of the present invention;

FIG. 3 shows schematically a second embodiment of the present invention;

FIG. 4 shows schematically a detail from the second embodiment of thepresent invention; and

FIG. 5 shows schematically the separator region of a third embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Well fluid received from an oil producing well typically contains morethan 10 vol % of highly dispersed water. The separation behaviour underthe influence of gravity of an oil/water dispersion containing more than10 vol % of water can be described by means of a model. Applicant haddeveloped the so-called Dispersion Band Model, see H. G. Polderman etal., SPE paper No. 38816, 1997. The model can be used to describeseparation in a separation chamber. An important mechanism of separationis based on coalescence of small water droplets in the dispersion band,which sink to the lower layer once the drops have grown large enough.During normal operation, three liquid layers are formed in theseparation chamber: a lower layer of relatively pure water, a middlelayer containing an oil and water dispersion and an upper layer ofrelatively dry oil. The middle layer is also referred to as thedispersion band.

A result of this model is an equation for the dispersion band thicknessH_(D) (m) as a function of the specific throughput Q/A (m/s), wherein Qis the volumetric flow rate through the separation chamber of the fluidto be separated (m³/s), and A is the horizontal cross-sectional area ofthe separation chamber (m²).

The relation between the dispersion band thickness H_(D) and thespecific throughput Q/A can be described by the equation that has beenexperimentally verified $\begin{matrix}{H_{D} = \frac{a\left( {Q/A} \right)}{1 - {b\left( {Q/A} \right)}}} & (1)\end{matrix}$

In this equation a and b are constants relating to the dispersionstability and they are a function of inter alia the kinematic viscosityof the oil component, the density difference between the oil and watercomponents, and the drop size distribution of the dispersion. For oilhaving a kinematic viscosity of 0.001 Pa.s a stable dispersion is forexample characterised by a=0.125 s, and b=0.078 s/m, whereas an unstabledispersion, which separates more readily, is for example characterisedby a=0.05 s, and b=0.032 s/m.

Reference is now made to FIG. 1, wherein curve A shows an example of thedispersion band thickness H_(D) (on the ordinate, in m) as a function ofthe specific throughput Q/A (on the abscissa, in m/s), calculated withequation (1). In the calculations a=0.05 s and b=0.032 s/m have beenused.

The dispersion band thickness H_(D) at a given volumetric flow rate Qand cross-sectional area A determines the minimum height that is neededfor a separation chamber in order that the upper oil layer and the lowerwater layer can be formed with the dispersion band between them.Similarly, an upper limit Q_(max) for the volumetric flow rate can becalculated by solving equation (1) for a given cross-sectional area andheight of the separation chamber, wherein it is assumed that H_(D) isequal to the height of the separation chamber. The upper limit Q_(max)divided by the volume of a separation chamber can be regarded as ameasure for the efficiency of the separation chamber.

It will now be shown, that the efficiency of a separation chamber can beincreased by installing a stack of vertically spaced apart inclinedplates. Such a stack of vertically spaced apart plates is also referredto as a plate pack.

A plate pack subdivides the separation chamber into a number ofseparation spaces, wherein the space delimited between two neighbouringplates is referred to as a separation space having a thickness H_(P)(m). In each separation space a dispersion band is formed, and theoverall thickness of the dispersion band is equal to the sum of thethickness of all individual dispersion bands. In a first approximation,the overall thickness of the dispersion band equals the height of theplate pack (n.H_(P)) needed to fully confine the dispersion. H_(D) canbe calculated by the following modification of equation (1):$\begin{matrix}{H_{D} = {{n \cdot H_{P}} = {n \cdot \frac{a\left( {Q/A} \right)}{n - {b\left( {Q/A} \right)}}}}} & (2)\end{matrix}$wherein H_(P) is the vertical distance between neighbouring plates (m),n is the number of plates arranged at equal vertical distance in theplate pack, and wherein the other symbols have the meaning givenhereinbefore.

Curve B in FIG. 1 has been calculated for a plate pack with H_(P)=0.3 m,using the same values for a and b as for the calculation of Curve A. AtQ/A=0.005 m/s the dispersion can be fully confined within 0.3 m, thuswithin a single pair of plates. At Q/A=0.020 m/s the dispersion can befully confined within 1.2 m, thus within a stack of 5 plates defining 4separation spaces of 0.3 m height each.

In contrast, curve A at 0.020 m/s gives a dispersion band thickness ofca. 2.7 m when no plate pack is used. This demonstrates that by using aplate pack a separation chamber of smaller height can handle the samespecific throughput as a larger separation chamber without a plate pack.

Reference is now made to FIG. 2, which shows schematically a firstembodiment of the present invention. The well 1, extending from thesurface 2 to the underground production formation 4, is provided with aseparation chamber 6 that is arranged in an underreamed section 7 of thewell 1. The separation chamber 6 has a substantially circular crosssection. The vertical-wall 8 of the separation chamber 6 is formed bythe surrounding formation 9, but it will be understood that the wall canalso be provided with a well tubular, such as a casing. The wall of theseparation chamber also forms the wall of the separator.

In the separation chamber 6 there is arranged an oil/water separator 10comprising an inlet 12 to receive well fluid from the inlet well section13 below the separation chamber 6. The separator 10 further comprises anoutlet 15 for an oil-enriched component opening into the well section 16above the separation chamber 6 and an outlet 18 opening into a dischargewell section 19 below the-separation chamber. The discharge well section19 communicates with a water discharge system. The water dischargesystem comprises in this example a discharge well 20 that is providedwith outlet means 21 to an underground formation 22 and a pump 23. Thewater discharge system further comprises means to prevent water fromflowing back into the separation chamber (not shown).

The separation chamber 6 of the well 1 includes a static separator 10.The static separator 10 comprises a flow distribution means 24, whichflow distribution means 24 comprises a vertical inlet conduit 25 havingan inlet at its lower end in communication with the inlet 12 for wellfluid of the static separator 10. The flow distribution means 24 furthercomprises an outlet conduit 26, which is in communication with the upperend of the inlet conduit 25. The outlet conduit 26 is provided with anumber of outlet openings 27 that open into the separation chamber 6 atsubstantially the same vertical position. A level detector means 28 isarranged to detect the level of an interface between liquid layers, withadvantage the level between the lower and middle layers. A signalgenerated by the level detector means 28 can with advantage be used tocontrol the flow of the inflowing well fluid, the outflowingwater-enriched component or the outflowing oil-enriched component independence on the measured vertical position. For example, the pump rateof a pump 23 of the water discharge system, which discharges thewater-enriched component received at the outlet 18, can be controlled inorder to keep the vertical position of the interface between the lowerand middle layers within predetermined limits.

During normal operation a well fluid comprising a mixture of oil andwater is received from the underground formation 4 through inlet means 3and flows along the well 1. The well fluid present in the inlet wellsection 13 below the separation chamber can be well fluid as directlyproduced from the underground formation 4, or can represent a streamobtained after a primary separation, for example a component obtainedafter bulk water removal in a horizontal well section. Preferably, thewell fluid entering the separator 10 at the inlet 12 contains between 10vol % and 80 vol % of water.

The well fluid is received by the inlet conduit 25 from the inlet 12.The well fluid is admitted into the separation chamber via openings 27at a predetermined vertical position. In this way, a relatively equaldistribution of the well fluid over the cross-sectional area of theseparation chamber is achieved which is advantageous for an efficientseparation. In particular, the local flow velocity of the inflowing wellfluid can be kept below 1 m/s, which is a critical value for most wellfluids under practical conditions above which no efficient separationcan be achieved. A lower layer of a water-enriched component will beformed, separated by an interface from a middle layer of water and oildispersion (the dispersion band). The vertical position of the interfacecan be measured by the level detector means 28, this measurement can beused to control the rate of disposal through the outlet 18, and in thisway the level of the interface can be regulated within predeterminedlimits. It can be chosen to arrange the interface just above, or below,the vertical position of the outlets from the flow distribution means24.

On top of the dispersion band an upper layer of an oil-enrichedcomponent is formed. The oil-enriched component flows to the outlet 15and on to the surface from where it is discharged at the wellhead (notshown). The oil-enriched component contains typically less than 10 vol %of water, preferably less than 2 vol %, more preferably less than 0.5vol % of water.

The water-enriched component flows to the outlet 18 from where it isdischarged via the water discharge system. The water-enriched componentcan contain between 0.01 vol % and 0.5 vol % of oil.

The outlet 15 is arranged to withdraw liquid from the region within theseparation chamber 6, wherein during normal operation the upper layer isformed, and the outlet 18 is arranged to withdraw liquid from the regionwherein the lower layer is formed. Preferably, like in this embodiment,the outlet 15 is arranged to withdraw fluid from the uppermost region ofthe separation chamber and outlet 18 is arranged to withdraw fluid fromthe lowermost region, so that the full physical height of the separationchamber is utilized.

The separation chamber 6 is so large that the dispersion band that isformed during normal operation fully fits into the chamber 6. Suitably,the ratio of the height to the effective diameter of the separationchamber is smaller than 10, preferably smaller than 5, wherein theeffective diameter is defined as the diameter of a circle having thesame cross-sectional area as the separation chamber.

It will be clear, that one or more outlet conduits of the fluiddispersion means 24 can be arranged in the form of a spider-likearrangement or a ring-like arrangement. Preferably, the outlet openingsare arranged such that they admit the fluid into the separation chamberhorizontally and tangentially with respect to the outer wall 8.

Reference is now made to FIGS. 3 and 4, which show a second embodimentof the present invention. In this embodiment, the static separator 10further comprises a stack of inclined, substantially flat plates 30, 31,32 that are arranged substantially parallel to each other and verticallyspaced apart at an equal distance. The space delimited between twoneighbouring plates is referred to as the separation space. For example,plates 30 and 31 define the separation space 35, plates 31 and 32 definethe separation space 36. Underneath the lowest plate 32 of the stack ofplates a parallel base plate 37 is arranged, wherein the outer rim ofthe base plate sealingly engages the walls of the separation chamber 6.Between the plate 32 and the base plate 37 a further separation space 38is defined.

The stack of plates is traversed by the inlet conduit 40, which extendsvertically upwardly from an opening 42 through the stack of plates inthe centre of the separation chamber 6. The passage of the inlet conduitthrough a plate, for example the passage 43 through plate 31, is therebyarranged such that the wall of the inlet conduit 40 sealingly fits tothe plate, for example plate 31, thereby preventing fluid communicationbetween neighbouring separation spaces, for example separation spaces 35and 36, along the inlet conduit. Further, the inlet conduit 40 isprovided with radial outlet openings 44, 45, 46, which open into theseparation spaces 35, 36, 38, respectively. It will be clear, thatfurther outlet openings can be arranged opening into different radialdirections. An outlet opening is with advantage arranged in thedirection of the axis in the horizontal plane around which the platesare inclined, i.e. in FIG. 2 an axis perpendicular to the paper plane.

Further details about the inclined plates will now be discussed withreference to FIG. 4, wherein schematically the plates 31 and 32 of FIG.3 are shown. The rim 47 of plate 31 includes at the upper side 48 of theplate 31 a straight edge 49 to which an upward pointing baffle plate 50is attached. At the lower side 52 the rim 47 includes a straight edge 54to which a downward pointing baffle plate 56 is attached.

Referring again to FIG. 3, the other inclined plates, of the stack ofplates are similarly provided with upward and downward pointing baffles58, 59, 60, 61 at the their upper and lower sides, respectively. Theremaining parts of the rim of each inclined plate to which no baffle isattached are arranged to sealingly engage the wall 8.

The static separator 10 further comprises an oil collection channel 65,which is formed by the space segment delimited by the upward pointingbaffles, 58, 50, 59, and the wall 8. The oil collection channel 65comprises oil inlets, for example oil inlet 70 arranged to receive fluidfrom the uppermost region 72 of the separation space 36. Oil inlet 70 isdefined by the upper edge 49 of the plate 31 and the upward pointingbaffle 59 of the plate 32 immediately below the oil inlet 70. The oilcollection channel 65 further comprises an outlet 73 in communicationwith the outlet 15 of the static separator 10.

Opposite to the oil collection channel 65 the separator 10 comprises awater collection channel 75, which is formed by the space segmentdelimited by the downward pointing baffles, 60, 56, 61, and the wall 8.The water collection channel 75 comprises water inlets, for examplewater inlet 80 arranged to receive fluid from the lowermost region 82 ofthe separation space 35. Water inlet 80 is defined by the lower edge 54of the plate 31 and the downward pointing baffle 60 of the plate 30immediately above the water inlet 80. The water collection channel 75further comprises an outlet 83 in communication with the outlet 18 ofthe separator 10.

The plates 30, 31 and 32 with the attached baffles are arranged suchthat the shortest horizontal distance between an upward pointing baffleand the wall 8 increases from bottom to top, and that the shortesthorizontal distance between a downward pointing baffle and the wall 8increases from top to bottom. In this way the cross-sectional areas ofboth the oil collection channel 65 and the water collection channel 75increase in the direction towards their respective outlets 73 and 83.Since the separator 10 does not contain parts that are moving duringnormal operation it represents a static oil-water separator.

During normal operation a well fluid comprising oil and water isreceived from the underground formation 4 through inlet means 3 and flowalong the well 1. The well fluid present in the inlet well section 13below the separation chamber can be well fluid as directly produced fromthe underground formation 4, or can represent a stream obtained after aprimary separation, for example a component obtained after bulk waterremoval in a horizontal well section. Preferably, the well fluidentering the static separator 10 at the inlet 12 contains between 10 vol% and 80 vol % of water. The well fluid then enters the inlet conduit 40at the opening 42 and is admitted into the interior of the separationspaces 35, 36, 38 via the outlet openings 44, 45 and 46. It has beenfound that good separation results are obtained if all openings have thesame cross-sectional area. Good results have further been obtained ifthe diameter of the openings is of the order of the diameter of theinlet conduit, such that the pressure drop over the opening is small.

The separation will now be discussed. To this end we take a closer lookon the separation space 36 between plates 31 and 32. In this separationspace 36, three liquid layers are formed, an upper, oil-enriched layer,a middle dispersion band layer and a lower, water-enriched layer. Theoil-enriched layer flows towards the uppermost region 72 of theseparation space 36, from where it leaves the separation space to enterthe oil collection channel through inlet 70. The water-enriched layerflows towards the lowermost region 85 of the separation space 36, fromwhere it enters the water collection channel through inlet 86.Separation in the spaces 35 and 38 is similar.

The oil collection channel 65 receives an oil-enriched component fromall separation spaces, and since the cross-section of the channel widenstowards the outlet 73, the vertically upward flow velocity of theoil-enriched component in the channel 65 can remain substantiallyconstant. From the outlet 73 the collected oil-enriched component flowsto the outlet 15 above the stack of plates, and on to the surface fromwhere it is discharged at the wellhead (not shown). The oil-enrichedcomponent contains typically less than 10 vol % of water, preferablyless than 2 vol %, more preferably less than 0.5 vol % of water.

The water-collection channel 75 receives a water-enriched component fromall separation spaces, and since its cross-section widens from top tobottom towards the outlet 83, the vertically downward flow velocity ofthe water-enriched component in the channel 75 can remain substantiallyconstant. From the outlet 83 the collected water-enriched componentflows to the outlet 18 below the stack of plates, from where it isdischarged via the water discharge system. The water-enriched componentcan contain between 0.01 vol % and 0.5 vol % of oil.

The height of the separation chamber 6, i.e. the shortest verticaldistance between the outlet for the oil-enriched component 15 and theoutlet for the water-enriched component 18, in this embodiment coincideswith the physical height of the separation chamber 6 in the underreamedsection 7. The stack of plates in the separation chamber is arranged tofully confine the dispersion during normal operation, such that theregion of the separation chamber above the stack of plates is filledwith the oil-enriched component, and the region below the stack ofplates is filled with the water-enriched component. As discussed withreference to FIG. 1, the height of the stack of plates can in firstapproximation be considered as the thickness of the dispersion band,since it is an upper limit for the sum of the thickness of allindividual dispersion bands in the separation spaces.

Reference is now made to FIG. 5. A further embodiment of a well 100according to the present invention will now be described. FIG. 5 showsschematically the separation chamber 6 of the well 100. Parts that aresimilar to parts discussed with reference to FIG. 3 are referred to withthe same reference numerals.

The inclined plates 130, 131 and 132, which form the stack of plates ofthe static separator 110, have the shape of funnels with substantiallycircular cross-section. The funnels in this embodiment are arranged suchthat they are narrowing from top to bottom. The funnels 130, 131 and 132are stacked parallel to each other at equal distance and substantiallyalong the central axis 133 of the separation chamber 6. Each funnel isprovided with a central opening, 140, 141, and 142.

The space delimited between two neighbouring funnels is referred to as aseparation space, FIG. 5 shows separation spaces 144 and 145. Underneaththe lowest plate 132 of the stack of plates a horizontal, flat baseplate 147 is arranged, wherein the outer rim of the base plate sealinglyengages the walls of the separation chamber.

The stack of plates is traversed by the inlet conduit 150, which extendsvertically upwardly from an opening 152 through the central opening ofeach of the funnels. The inlet conduit 150 comprises outlet conduits154, 155, 156, 157. Each of the outlet conduits extends into theinterior of a separation space where it is provided with an outletopening, outlet openings 158, 159, 160, 161. It will be clear, thatfurther outlet conduits and openings can be arranged opening intodifferent directions.

To the whole rim of the central opening of each funnel a downwardpointing baffle is attached, and to the whole upper rim of each funnelan upward pointing baffle is attached. The downward pointing baffles areschematically shown with reference numerals 170, 171, 172, and theupward pointing baffles with numerals 174, 175, 176. The oil collectionchannel 178 is formed by the annular space delimited by the upwardpointing baffles 174, 175, 176 and the wall 8. Oil inlets 181, 182 tothe oil collection channel 178 are defined by the annular regionsbetween an upward pointing baffle 175, 176 and the upper rim of theupper adjacent funnel, 130, 131, respectively. For example, oil inlet181 is arranged to receive an oil-enriched component from the uppermostregion 183 of the separation space 145. The oil collection channel 178further comprises an outlet 184 in communication with the outlet 15 ofthe separator 110.

The water collection channel 180 of the separator 110 is formed by thenear-axial space delimited by the downward pointing baffles 170, 171,172. Water inlets 186, 187 to the water collection channel 180 aredefined by the annular regions between a downward pointing baffle 170,171 and the rim of the adjacent circular opening, 141, 142,respectively. For example, water inlet 187 is arranged to receive awater-enriched component from the lowermost region 189 of the separationspace 145. The water collection channel 180 further comprises an outlet190 in communication with the outlet 18 of the separator 110.

The diameter of the upper rim increases from top to bottom, such thatthe cross-sectional area of the oil collection channel 178 increasestowards the outlet 184. The cross-sectional area of the centralopenings, and therefore of the water-collection channel, increases fromtop to bottom, i.e. towards the outlet 190. The lowest downward baffle172 close to the outlet 190 of the water collection channel traversesthe base plate 147, wherein the outer circumference of the baffle 172sealingly engages the base plate 147. The outlet 190 is communicatingwith the separator's outlet for the water-enriched component via conduit192 which is attached to the lower rim of the downward baffle 172. Inthe transition wall 193 the opening 152 is arranged to which the inletchannel 150 is attached.

For the discussion of normal operation of the well 100 of thisembodiment reference is made to the normal operation of the embodimentdiscussed with reference to FIGS. 2 and 3. In the following only theoperation of the separator 110 will be discussed.

Well fluid is received by the static separator 110 in the same way atthe inlet 12, and enters the inlet conduit 150 at the opening 152. Thewell fluid is admitted into the interior of the separation spaces 144,145 via the outlet openings 158, 159, 160, 161. In a separation space,for example separation space 145, an upper, oil-enriched layer and alower, water-enriched layer are formed. For example, in separation space145 the oil-enriched layer flows towards the uppermost region 183, fromwhere it leaves the separation space to enter the oil collection channelthrough inlet 181. The water-enriched layer flows towards the lowermostregion 189 of the separation space 145, from where it enters the watercollection channel through inlet 187. The oil-collection channel 178receives an oil-enriched component from all separation spaces, and sincethe cross-section of the channel widens towards the outlet 184, thevertically upward flow velocity of the oil-enriched component in thechannel 178 can remain substantially constant. From the outlet thecollected oil-enriched component flows to the outlet 15. Theoil-enriched component contains typically less than 10 vol % of water,preferably less than 2 vol %, more preferably less than 0.5 vol % ofwater.

The water-collection channel 180 receives a water-enriched componentfrom all separation spaces, and since its cross-section widens from topto bottom towards the outlet 190, the vertically downward flow velocityof the water-enriched component in the channel 180 can remainsubstantially constant. From the outlet 190 the collected water-enrichedcomponent flows to the outlet 18 from where it is discharged via thewater discharge system. The water-enriched component can contain between0.01 vol % and 0.5 vol % of oil.

The baffles along the water and oil collection channels can be regardedas serving different purposes. They enclose the well fluid in theseparation spaces such that the separation spaces can be regarded asbeing effectively decoupled. Further, the baffles prevent remixing of analready separated component in a collection channel with the fluid in aseparation space, considering that the flow velocities in the collectionchannels are relatively high. The baffles help to realise that thevertical flows of inflowing well fluid and outflowing separatedcomponents are effectively decoupled.

It will be understood that one modification of the separator 110 shownin FIG. 5 can be obtained by arranging the stack of funnels upside downsuch that they are narrowing from bottom to top, and it will be clearthat and how in such an arrangement the oil collection channel is formedin the near-axial region and the water-collection channel in the annularregion of the separation chamber.

Another modification of the separator 110 can be obtained by sealinglyattaching parts of the upper rims of the funnels to the outer wall, suchthat one or more oil collection channels are formed in space segmentsalong the outer wall.

In yet another modification the inlet channel is arranged off-centre inthe separation chamber, and sealingly traverses the stack of platessimilar to the embodiment of the separator 10 in FIG. 3.

It will be clear that specific design parameters of a plate pack willdepend on the practical situation. For example, the cross sectional areaof the water collection and oil collection channels, relative to eachother and to the separation chamber's cross sectional area, can beselected depending on the expected flow rates and the water content ofthe well fluid. The number of plates can be selected on the basis ofcalculations similar to FIG. 1 using the parameters of the practicalsituation. The inclination angle of the plates with respect to thehorizontal plane is selected such that solid particles do not accumulateon the plates, but that the available separation volume is optimallyused. Typically the inclination angle would be selected in the rangebetween 10 and 45 degrees, preferably between 15 and 25 degrees, withrespect to the horizontal plane.

In the discussion with reference to FIG. 1 it has become clear, that astack of plates increases the separation efficiency of a separator in aseparation chamber. In practice often a reduction of the required heightof the separation chamber by a factor in the range of from 1.5 to 6 canbe achieved. Sometimes, the height of the separation chamber is not alimiting factor for the well design, and in this case a separatorwithout a stack of plates can be used.

Typical dimensions of the separation chamber 6 of the well as shown inFIG. 1 have been calculated using the Dispersion Band Model under thefollowing assumptions: gross flow rate through the separator 1000 m³/dayof well fluid containing 50 vol % of water, dry oil viscosity 0.001Pa.s. In this case a separation chamber of about 1 m diameter and 5 mheight is required. For comparison it is noted that by installing astack of plates in the separation chamber the height requirement can bedecreased to for example 2 m.

1. A well extending from the earth's surface to an undergroundproduction formation containing hydrocarbon oil and water, where thewell above the production formation is provided with a separationchamber in which a static oil/water separator is arranged, the staticseparator comprising: an inlet to receive well fluid from an inlet wellsection below the separation chamber; an outlet for an oil-enrichedcomponent opening into the well section above the separation chamber; anoutlet for a water-enriched component opening into a discharge wellsection below the separation chamber, a dispersion band that is formedtherein under normal operation conditions, wherein the height of theseparation chamber is larger than the thickness of the dispersion band;a stack of vertically spaced apart inclined plates, wherein between eachpair of neighbouring plates a separation space is defined; asubstantially vertical inlet conduit communicating with the separator'sinlet, where the inlet conduit traverses the stack of plates and isarranged to receive the well fluid at its lower end, and is providedwith one or more well fluid outlets each of which opens into aseparation space; a substantially vertical oil collection channel havingan oil outlet at its upper end communicating with the separator's outletfor the oil-enriched component, where the oil collection channel has oneor more oil inlets, each oil inlet being arranged to receive fluid fromthe uppermost region of a separation space, wherein at least the plateimmediately below each oil inlet is provided with a vertically upwardpointing baffle; and a substantially vertical water collection channelhaving a water outlet at its lower end communicating with the separatorsoutlet for the water-enriched component, where the water collectionchannel has one or more water inlets, each water inlet being arranged toreceive fluid from the lowermost region of a separation space, whereinat least the plate immediately above each water inlet is provided with avertically downward pointing baffle.
 2. The well according to claim 1,wherein the static separator further comprises a flow distributor means,arranged to distribute at a predetermined vertical position the wellfluid received through the separator's inlet over the cross-sectionalarea of the separation chamber.
 3. A well according to claim 2, whereinthe flow distributor means comprises one or more conduits in fluidcommunication with the separator's inlet for well fluid, which conduitsare provided with outlet openings near the predetermined verticalposition into the separation chamber.
 4. The well according to claim 1,wherein the static separator further comprises a level detector meansand a flow control means in order to maintain an interface between twoliquid layers at a predetermined level during normal operations.
 5. Thewell according to claim 1, wherein the inclined plates are substantiallyflat and arranged substantially parallel to each other, wherein eachinclined plate is provided with a downward pointing baffle attached tothe rim at the lower side of the inclined plate and an upward pointingbaffle attached to the rim at the upper side of the inclined plate,wherein the remaining parts of the rim fit sealingly to the wall of theseparation chamber, wherein the oil collection channel is formed by thespace delimited by the upward pointing baffles and the wall, and whereinthe water collection channel is formed by the space delimited by thedownward pointing baffles and the wall.
 6. The well according to claim1, wherein the inclined plates have substantially the form of funnelsarranged substantially parallel to each other, wherein each funnel isprovided with a central opening.
 7. The well according to claim 6,wherein the funnels are narrowing from top to bottom, wherein a downwardpointing baffle is attached, and wherein an upward pointing baffle isattached to the upper rim, wherein the water collection channel isformed by the axial space delimited by the downward pointing baffles,and wherein the oil collection channel is formed by the annular spacedelimited by the upward pointing baffles and the wall.
 8. The wellaccording to claim 6, wherein the funnels are narrowing from bottom totop, wherein an upward pointing baffle is attached to the rim of eachcentral opening, and wherein a downward pointing baffle is attached tothe lower rim, wherein the oil collection channel is formed by the axialspace delimited by the upward pointing baffles, and wherein the watercollection channel is formed by the annular space delimited by thedownward pointing baffles and the wall.
 9. The well according to claim1, wherein the cross-sectional area of the water collection channelincreases from top to bottom.
 10. The well according to claim 1, whereinthe cross-sectional area of the oil collection channel increases frombottom to top.
 11. The well according to claim 1, wherein the outletopenings of the inlet channel are the same size.
 12. A method ofproducing oil from an underground production formation through a wellcontaining hydrocarbon oil and water, where the well above theproduction formation is provided with a separation chamber in which astatic oil/water separator is arranged, the static separator comprising:a. an inlet to receive well fluid from an inlet well section below theseparation chamber; b. an outlet for an oil-enriched component openinginto the well section above the separation chamber; c. an outlet for awater-enriched component opening into a discharge well section below theseparation chamber, d. a dispersion band that is formed therein undernormal operation conditions, wherein the height of the separationchamber is larger than the thickness of the dispersion band; e. a stackof vertically spaced apart inclined plates, wherein between each pair ofneighbouring plates a separation space is defined; f. a substantiallyvertical inlet conduit communicating with the separator's inlet, wherethe inlet conduit traverses the stack of plates and is arranged toreceive the well fluid at its lower end, and is provided with one ormore well fluid outlets each of which opens into a separation space; g.a substantially vertical oil collection channel having an oil outlet atits upper end communicating with the separator's outlet for theoil-enriched component, where the oil collection channel has one or moreoil inlets, each oil inlet being arranged to receive fluid from theuppermost region of a separation space, wherein at least the plateimmediately below each oil inlet is provided with a vertically upwardpointing baffle; and h. a substantially vertical water collectionchannel having a water outlet at its lower end communicating with theseparator's outlet for the water-enriched component, where the watercollection channel has one or more water inlets, each water inlet beingarranged to receive fluid from the lowermost region of a separationspace, wherein at least the plate immediately above each water inlet isprovided with a vertically downward pointing baffle; comprising thesteps of: a. admitting well fluid into the separation chamber at apredetermined vertical position through one or more openings at a localflow velocity below 1 m/s; b. allowing the well fluid to separate into alower layer of a water-enriched component, a middle layer of an oil andwater dispersion component and an upper layer of an oil-enrichedcomponent, c. withdrawing liquid from the upper layer and producing thisliquid to the surface; d. withdrawing liquid from the lower layer; e.measuring the vertical position of the interface between two liquidlayers; and f. controlling the flow rate of at least one of theinflowing well fluid, the outflowing water-enriched component or theoutflowing oil-enriched component in dependence on the measured verticalposition.
 13. A method according to claim 12, including the step ofcontrolling the flow rate to arrange the predetermined vertical positionin the lower layer.
 14. A method according to claim 12, including thestep of controlling the flow rate to arrange the predetermined verticalposition in the middle layer.